The present invention relates to an exhaust gas purification system and an exhaust gas purification method for an engine.
Exhaust gas from diesel engines or lean-burn gasoline engines contains nitrogen oxides (NOx) and particulates. Japanese Unexamined Patent Publication No. 2011-89521 discloses a system for treating NOx and particulates. This system makes exhaust gas pass through an oxidation catalyst, a particulate filter, a source of a reducing agent, a selective catalytic reduction (SCR) catalyst, and an NH3 oxidation catalyst, in this order. In this system, the oxidation catalyst oxidizes NO in the exhaust gas and thereby produces NO2, and the particulates on the filter are combusted in the presence of the produced NO2, thereby reconditioning the filter. As the reducing agent, NH3 or urea is supplied to an exhaust gas passage, and the SCR catalyst selectively reduces and purifies the NOx. NH3 and/or derivatives thereof that have/has passed through the SCR catalyst are/is removed by the NH3 oxidation catalyst.
Japanese Unexamined Patent Publication No. H09-53442 discloses a system in which exhaust gas is made to pass through an oxidation catalyst, a particulate filter, and a NOx trap catalyst, in this order. In this system, like the system of Japanese Unexamined Patent Publication No. 2011-89521, the oxidation catalyst oxidizes NO in the exhaust gas and thereby produces NO2, and the particulates collected by the filter are combusted in the presence of the produced NO2. NO produced through the reaction of the NO2 and the particulates and NO2 that has passed through the filter without reacting with the particulates are trapped by the NOx trap catalyst. The trapped NOx is released by cyclically richening the air-fuel ratio of the exhaust gas by fuel injection in exhaust strokes. The released NOx is then reduced and purified.
Japanese Unexamined Patent Publication No. H09-53442 also discloses a system which includes a NOx selective reduction catalyst, instead of a NOx trap catalyst. In this system, the air-fuel ratio of the exhaust gas is cyclically richened by fuel injection in exhaust strokes so as to increase the amounts of hydrocarbon (HC) and CO in the exhaust gas. The HC and other substances are accumulated in the NOx selective reduction catalyst (SCR catalyst), and NO and NO2 are reduced and purified downstream from the filter by using the accumulated HC.
In each of the systems of the above patent publications, since the particulates are combusted by using the NO2 produced by the oxidation catalyst, the filter is reconditioned at a relatively low temperature. This allows for reducing or omitting post injection which is performed to raise the temperature of the filter. (The post injection is to inject fuel into the combustion chamber of an engine in expansion strokes or exhaust strokes. The post injection increases the amounts of HC and CO in exhaust gas, and heat of oxidation reaction of the HC and CO in an oxidation catalyst raises the temperature of the exhaust gas.)
As can be seen from the foregoing, according to the known systems, the use of NO2, while preventing the reconditioning of the filter from lowering the fuel efficiency of the engine, causes an increase in the amount of NOx which should be reduced and purified by the SCR catalyst and the NOx trap catalyst that are arranged downstream from the filter. Therefore, according to the system of Japanese Unexamined Patent Publication No. 2011-89521, it is necessary to increase the capacity of its reducing agent tank so as to supply the SCR catalyst with a sufficient amount of the reducing agent (NH3 or urea). However, in a compact car, for example, it is difficult to ensure a space where a reducing agent tank having such a large capacity can be installed. Further, if an increased amount of the reducing agent is used, the amount of NH3 passing through the SCR catalyst increases, and a heavier load is placed on the NH3 oxidation catalyst that treats NH3. The system of Japanese Unexamined Patent Publication No. H09-53442, in which HC is used as a reducing agent for the SCR catalyst, does not need a reducing agent tank. According to this system, however, it is necessary to frequently bring the air-fuel ratio of the exhaust gas into a condition close to rich, and therefore, fuel efficiency is lowered due to the reduction of NOx. Even if a NOx trap catalyst is used, instead of the SCR catalyst, to reduce NOx, it is necessary to frequently richen the air-fuel ratio of the exhaust gas. Thus, in this case, the fuel efficiency is also lowered.